1. Field of the Invention
This invention relates to various types of variable capacity type radial flow turbines for a turbocharger and so forth in which an inletting cross sectional area thereof can be changed.
2. Description of the Prior Art
The conventional type of the variable capacity turbine for a turbocharger will now be described with reference to FIGS. 12 and 13. A turbine wheel 320 is disposed in a housing 321 which forms an exhaust gas passage 327 which accelerates the exhaust gas which has been introduced. A pivotable vane 323 which is disposed in a portion 326 through which the exhaust gas is introduced into the turbine wheel 320 is opened and closed, whereby the turbine geometry is varied. In this case, as shown in FIG. 13, the passage throat area becomes A.sub.1 when the pivotable vane 323 is closed, while the passage throat area becomes A.sub.2 when the pivotable vane 323 is opened. As mentioned above, the throat area of the passage is changed, and this change causes the accelerating ratio to be changed, whereby the turbine capacity is changed.
Another type of the conventional variable capacity turbine is shown in FIGS. 14 and 15. In the variable inlet port type radial flow turbine, shown in FIGS. 14 and 15, the gas introduced through an inlet port of a scroll passage 400 flows through a passage 430 which is formed by a pivotable flap vane 420 and an inner wall 401 of the scroll passage, and the gas is then introduced into a rotating blade 440 through the inner side of a rear scroll passage 402.
A rotary shaft 422 which is disposed in the front edge portion 421 of the flap vane 420 projects outside through a penetrating hole 403 in the wall adjacent to the scroll passage 400. The flap vane 420 is therefore capable of being pivoted relative to the axis of the rotary shaft 422 as illustrated by the short dash line by turning a lever 423 provided with a handle of the rotary shaft 422.
By rotating the flap vane 420 relative to the axis of the rotary shaft 422, the distance between the inner wall 401 and a rear end 424 of the flap vane 420 is changed, whereby flow through area of the passage 430 is changed for the purpose of changing the flow characteristics of the turbine.
In the conventional type variable capacity turbine having a pivotable vane, shown in FIGS. 12 and 13, the amount of the exhaust gas at the time when the vane is opened and which is allowed to be introduced into the turbine wheel, and the range of amount of the gas which is between the throat area A.sub.2 and the throat area A.sub.1, is defined in accordance with the length of the pivotable vane 323. Therefore, the variable range of the geometry of the turbine can be made large by lengthening the pivotable vane 323, but operation of the long pivotable vane in the atmosphere of high temperature and an exhaust gas causes the durability to deteriorate. If the pivotable vane is lengthened, the angle at the time of opening and closing the vane is not changed, therefore the distance of shifting the tip of the pivotable vane becomes large in accordance with the length of the pivotable vane. The turbine performance sometimes deteriorates because the vane transverses the exhaust gas flow when the pivotable vane is opened.
The conventional type of the variable inlet port radial flow turbine shown in FIGS. 14 and 15 is a type in which the flap vane 420 is pivoted relative to the axis of the rotary shaft 422 which is disposed at the front end portion 421 of the flap vane 420 for the purpose of changing the area of the passage 430 which is formed by the rear end 424 of the flap vane 420 and the inner wall 401 of the scroll passage. Therefore, when the turbine flow rate is intended to be reduced, the rear end 424 of the flap vane 420 must be brought to near the inner wall 401 of the scroll passage. As a result of this, a dead water region is generated in the rear stream or downstream side of the flap vane 420, whereby the efficiency of the turbine rapidly deteriorates.
In the case where the flow rate of the turbine is intended to be increased in the conventional type of the variable inlet port type radial flow turbine, the rear end 424 of the flap vane 420 must be brought to a position far from the inner wall 401 of the scroll passage so as to expand the passage 430. In this case, a certain distance must be kept between the rear end 424 and the rotating blade 440 for the purpose of preventing interference. If the area of the passage 430 is intended to be increased for the purpose of increasing the maximum flow rate of the turbine with respect to the inner wall 401 of the scroll passage, the rear end 424 of the pivotable vane 420 must therefore be brought to the radially innermost position. In this case, when the flow rate is intended to be reduced, the rotational angle .theta. of the flap vane 420 must be further increased, whereby the dead water region which is generated at a region downstream of the flap vane 420 becomes large, as a result of which, the efficiency deteriorates.